As semiconductor fabrication strides toward features of ever smaller dimensions, the requisite resolution tolerances of processing steps become more stringent apace. Technology is near the point at which prior art methods no longer suffice to the task.
As background, salient components of a prior art lithographic projection system, designated generally by numeral 100, to which the present invention is applicable, are now described with reference to FIG. 1. Lithographic projection system 100, in its entirety, may be referred to herein, synonymously, as a “lithographic tool,” a “microlithographical tool,” or as a “litho tool.” Lithographic projection system 100 is used for fabrication of integrated circuits using CMOS (complementary metal-oxide-semiconductor) or kindred lithographic technologies.
During the course of operation of lithographic projection system 100, wafers 105, or other substrates, are successively stepped by motion of wafer stage 107 such that a portion, or all, of a wafer substrate 105 falls within an intense electromagnetic beam 109, which may be referred to herein simply as beam 109. Beam 109 is generated by illumination source (or “light source,” or “source”) 101 of the lithographic projection system 100, where illumination source 101 is a source of substantially monochromatic radiation such as an excimer laser, or other laser, or other deep ultraviolet (DUV) source, although the wavelength range of source 101 is not limited, for purposes of the present description and claims, and source 101 may also be an x-ray source, or a source in the extreme ultraviolet (EUV), all within the scope of the present invention.
Electromagnetic radiation 110 emitted by source 101 may be referred to herein as “light,” without loss of generality, understanding that the term “light” similarly encompasses x-rays and other electromagnetic radiation. An aperture 112 and condenser lens system 114 define the étendue of the projection system 100, while reduction lens system 118 defines the numerical aperture (NA) of beam 109 focused on or near wafer substrate 105. A photomask 116 (which may be also referred to herein as a “reticle”), is interposed between condenser lens system 114 and reduction lens system 118, and includes one or more patterns or targets 120 defined on its surface. Reticle 116, containing a die or an array of dies, is projected onto wafer substrate 105, which is then stepped for subsequent repetition of the projection pattern onto the wafer. Reticle 116 typically comprises chromium patterns on a quartz plate, generally called chrome-on-glass (COG) technology. Projection system 100 is characterized by an optical axis 180 (shown in FIG. 1) that defines the direction of propagation of electromagnetic beam 110 and that is transverse to wafer substrate 105.
FIG. 2 shows a prior art immersion scanner projection system, designated generally by numeral 200. Immersion scanner projection system 200 is a particular case of projection system 100, and contains identical components, other than that the space between condenser lens system 118 and wafer substrate 105 contains a fluid 205, typically water, allowing higher numerical apertures, and thus tighter focus and higher feature resolution, to be achieved.
Using either the dry scanner of lithographic projection system 100 or the immersion scanner projection system 200, beam 109 serves to pattern openings in a photoresistive polymer deposited on wafer substrate 105. When the photoresistive polymer is developed and rinsed, an indicated etch, deposit or implanting of material may be made onto, or into, the wafer substrate.
It is critical that the pattern encoded in the photomask 116 be accurately focused at a specified plane relative to wafer stage 107. As successive layers are built up through a series of deposition steps, the optimum focal plane rises to a successively higher position of the photoresist above the base of wafer substrate 105 at wafer stage 107. Thus the location of the focus of beam 109 must be known to a degree of accuracy commensurate with the focal depth of the beam. Various phenomena such as non-flatness of the ensemble of wafer substrates 105, or resist thickness variations, may give rise to effective changes of focus, which must be tracked and cured to achieve optimal feature resolution.
In order to quantify any focus offset, prior art practice has taught techniques whereby a phase-shifting mask structure provides for monitoring focus. Such a prior art phase-shifting mask structure 300 is shown in FIG. 3A and is described in U.S. Pat. No. 5,300,786 (to Brunner et al., hereinafter “Brunner '786”), which is incorporated herein by reference in its entirety. Phase-shifting mask structure 300, as applied for monitoring focus, may be referred to as a Phase Shift Focus Monitor, or “PSFM.” Prior art phase-shifting mask structure 300 has a configuration of opaque line segments 304 that are either horizontal or vertical in the plane of a surface of a substrate 302, where the substrate is a transparent material such as quartz. In the prior art embodiment of FIG. 3A, opaque line segments 304 (or “shield lines”) do not intersect with one another. The prior art phase-shifting mask structure 300 has phase shift windows 306 where the optical depth through the window, ∫n(z)dz, where z is the coordinate into the plane of the surface of substrate 302, and n(z) is the index of refraction, differs from the optical depth through a non-phase shifted zone 308 of substrate 302, typically by an amount corresponding to a fraction of a wavelength of electromagnetic radiation 110.
In the prior art phase-shifting mask structure 300, phase-shifted windows 306 are rectangular, and either adjacent or non-adjacent, and triangular. By projecting prior art phase-shifting mask structure 300 onto surface 106 of wafer substrate 105 (shown in FIG. 1), a (reduced) image 340 is formed as shown in FIG. 3B. Surface 106 may also be referred to herein as the “plane of the projected image” since it is the focal plane of the optical system of which the reticle is placed in the object plane. Measurement of displacement between an inner “square” 342 and an outer “square” 344 yields an “overlay” that, once calibrated, provides a measure of litho tool focus.
Other configurations of phase shift test patterns have been taught in the art, such as that shown in FIG. 3C (originally FIG. 8 of Brunner '786). In the prior art embodiment shown in FIG. 3C, an outer square 351 of shield lines 304 functions as a control to confirm the extent and direction of line shift resulting from a phase shift region 358 between a middle square 353 and an inner square 355 of shield lines 304.
A problem arising in the prior art phase shift test patterns is that of the appearance of a ghost image, described with reference to FIGS. 4A-4C, which show the resultant projection image 360 resulting from projection of the phase-shifting mask structure 300 shown in FIG. 3A under conditions of successively lower illumination dose. As the dose decreases, ghost image (GI) 362 appears as a line along the diagonal edge of phase-shifted region 309 (shown in FIG. 3A). Ghost image 362 confounds the determination of overlay and thus of defocus. Ghost images are particularly problematic when immersion tools are used.
It would thus be desirable to provide a reticle that allows for more accurate and robust determination of focus. Moreover, it would be desirable to monitor other characteristics of the lithographic projection system 100 by means of a projected reticle. In particular, monitoring the polarization state of a high-NA immersion scanner has become more and more important as technology moves into, and beyond, 32-nm illumination. Previously suggested resist-based technologies for monitoring source polarization all require specially designed reticles, each of which contains multiple polarimeters, and also require a wide dose range, exceeding 300 mJ/cm2, and many scanning electron microscope (SEM) images, in order to obtain satisfactory measurements of source polarization. These include those of McIntyre et al., “PSM Polarimetry: Monitoring Polarization at 193 nm High-NA and Immersion with Phase Shifting Masks,” J. Microlithography, Microfabrication, and Microsystems, vol. 4, 031102 (2005), and Tu et al., “Resist-based Polarization Monitoring for 193 nm High-Numerical Aperture Lithography,” Proc. SPIE 7140, Lithography Asia 2008, 714019 (2008), and U.S. Pat. No. 8,679,708 (to Brunner et al.), all of which references are incorporated herein by reference in their entirety. It would, thus, be desirable to monitor the polarization state of a high-NA immersion scanner using a single target used also for focus monitoring.